One of the most important requirements of a ball joint, especially those used in automotive steering links and automotive suspension systems, is to maintain for a long service life a certain optimum rotating torque value and a certain optimum swivelling torque value.
However, in practice, due to the inevitably developing wear in the bearing members as the operating hours or the total mileage increases, the value of the bearing contact load decreases, and consequently the rotating and swivelling torque values are reduced, and the steering wheel play as well as the oscillation increases. In order to overcome the defect of the reduction of bearing contact load value as mentioned above in conventional ball joint design, a coil spring is disposed in the space formed by the socket cavity of a metallic socket joint rod and a metallic cover disc, or a plastic bearing block is forced into the space formed by the socket cavity of a metallic socket joint rod and a metallic cover disc so that the elasticity of the bearing block per se is utilized.
However, the ball joint arrangement that makes use of a coil spring is defective in that there is only a limited space to accommodate the coil spring. Further, when the apparent compressive longitudinal elastic modulus Ec is small (e.g. Ec=0.8 kg/mm.sup.2), the torque value cannot be set sufficiently high to meet the requirement of optimum rotating and swivelling torque values. Similarly, the ball joint arrangement that makes use of the elasticity of a plastic bearing block per se is also defective in that its compressive longitudinal elastic modulus Ep is too large (e.g., Ep=50 kg/mm.sup.2 for high density polyethylene). Not only is there a large reduction of bearing contact load value even with a small change of its dimensions due to the abrasion that accompanies a large reduction of rotating and swivelling torque values, but also, in the production process of a ball joint with a small deviation of the dimensions in the relevant members, there results in a large deviation of the initially set rotating and swivelling torque values. Therefore under such circumstances, ball joints can hardly be quantity-produced with a uniform torque value.